[gardening] Move switch building utility code from GenEnum.cpp to SwitchBuilder.h

This code is utility code that is just thrown around in the middle of the enum
emission code and makes it difficult to follow. It is something that is large
enough really to stand on its on.

Author: gottesmm

lib/IRGen/GenEnum.cpp

@@ -123,6 +123,7 @@
 #include "ResilientTypeInfo.h"
 #include "ScalarTypeInfo.h"
 #include "StructLayout.h"
+#include "SwitchBuilder.h"
 
 using namespace swift;
 using namespace irgen;
@@ -239,190 +240,6 @@ irgen::EnumImplStrategy::getResilientTagIndex(EnumElementDecl *Case) const {
 }
 
 namespace {
-  /// A builder that produces an LLVM branch or switch instruction for a set
-  /// of destinations.
-  class SwitchBuilder {
-  private:
-#ifndef NDEBUG
-    /// Track whether we added as many cases as we expected to.
-    unsigned CasesToAdd;
-#endif
-
-  protected:
-    IRBuilder Builder;
-    llvm::Value *Subject;
-    
-    /// Protected initializer. Clients should use the `create` factory method.
-    SwitchBuilder(IRGenFunction &IGF, llvm::Value *Subject, unsigned NumCases)
-      :
-#ifndef NDEBUG
-        CasesToAdd(NumCases),
-#endif
-        // Create our own IRBuilder, so that the SwitchBuilder is always able to
-        // generate the branch instruction at the right point, even if it needs
-        // to collect multiple cases before building an instruction.
-        Builder(IGF.IGM.getLLVMContext(), /*DebugInfo*/ false), Subject(Subject)
-    {
-      // Start our builder off at IGF's current insertion point.
-      Builder.SetInsertPoint(IGF.Builder.GetInsertBlock(),
-                             IGF.Builder.GetInsertPoint());
-    }
-    
-  public:
-#ifndef NDEBUG
-    virtual ~SwitchBuilder() {
-      assert(CasesToAdd == 0 && "Did not add enough cases");
-    }
-#endif
-
-    // Create a SwitchBuilder instance for a switch that will have the given
-    // number of cases.
-    static std::unique_ptr<SwitchBuilder>
-    create(IRGenFunction &IGF, llvm::Value *Subject,
-           SwitchDefaultDest Default,
-           unsigned NumCases);
-    
-    /// Add a case to the switch.
-    virtual void addCase(llvm::ConstantInt *value, llvm::BasicBlock *dest) {
-#ifndef NDEBUG
-      assert(CasesToAdd > 0 && "Added too many cases");
-      --CasesToAdd;
-#endif
-    }
-  };
-  
-  /// A builder that emits an unconditional branch for a "switch" with only
-  /// one destination.
-  class BrSwitchBuilder final : public SwitchBuilder {
-  private:
-    friend class SwitchBuilder;
-
-    BrSwitchBuilder(IRGenFunction &IGF, llvm::Value *Subject, unsigned NumCases)
-      : SwitchBuilder(IGF, Subject, NumCases)
-    {
-      assert(NumCases == 1 && "should have only one branch");
-    }
-    
-  public:
-    void addCase(llvm::ConstantInt *value, llvm::BasicBlock *dest) override {
-      SwitchBuilder::addCase(value, dest);
-      Builder.CreateBr(dest);
-    }
-  };
-  
-  /// A builder that produces a conditional branch for a "switch" with two
-  /// defined destinations.
-  class CondBrSwitchBuilder final : public SwitchBuilder {
-  private:
-    friend class SwitchBuilder;
-    llvm::BasicBlock *FirstDest;
-
-    CondBrSwitchBuilder(IRGenFunction &IGF, llvm::Value *Subject,
-                        SwitchDefaultDest Default,
-                        unsigned NumCases)
-      : SwitchBuilder(IGF, Subject, NumCases),
-        FirstDest(Default.getInt() == IsUnreachable
-                    ? nullptr : Default.getPointer())
-    {
-      assert(NumCases + (Default.getInt() == IsNotUnreachable) == 2
-             && "should have two branches total");
-    }
-    
-  public:
-    void addCase(llvm::ConstantInt *value, llvm::BasicBlock *dest) override {
-      SwitchBuilder::addCase(value, dest);
-      
-      // If we don't have a first destination yet, save it.
-      // We don't need to save the value in this case since we assume that the
-      // subject must be one of the case values. We only have to test one or
-      // the other.
-      //
-      // TODO: It may make sense to save both case values, and pick which one
-      // to compare based on which value is more likely to be efficiently
-      // representable in the target machine language. For example, zero
-      // or a small immediate is usually cheaper to materialize than a larger
-      // value that may require multiple instructions or a bigger encoding.
-      if (!FirstDest) {
-        FirstDest = dest;
-        return;
-      }
-      
-      // Otherwise, we have both destinations for the branch and a value to
-      // test against. We can make the instruction now.
-      if (cast<llvm::IntegerType>(Subject->getType())->getBitWidth() == 1) {
-        // If the subject is already i1, we can use it directly as the branch
-        // condition.
-        if (value->isZero())
-          Builder.CreateCondBr(Subject, FirstDest, dest);
-        else
-          Builder.CreateCondBr(Subject, dest, FirstDest);
-        return;
-      }
-      // Otherwise, compare against the case value we have.
-      auto test = Builder.CreateICmpNE(Subject, value);
-      Builder.CreateCondBr(test, FirstDest, dest);
-    }
-  };
-  
-  /// A builder that produces a switch instruction for a switch with many
-  /// destinations.
-  class SwitchSwitchBuilder final : public SwitchBuilder {
-  private:
-    friend class SwitchBuilder;
-    llvm::SwitchInst *TheSwitch;
-    
-    SwitchSwitchBuilder(IRGenFunction &IGF, llvm::Value *Subject,
-                        SwitchDefaultDest Default,
-                        unsigned NumCases)
-      : SwitchBuilder(IGF, Subject, NumCases)
-    {
-      TheSwitch = IGF.Builder.CreateSwitch(Subject, Default.getPointer(),
-                                           NumCases);
-    }
-    
-  public:
-    void addCase(llvm::ConstantInt *value, llvm::BasicBlock *dest) override {
-      SwitchBuilder::addCase(value, dest);
-      TheSwitch->addCase(value, dest);
-    }
-  };
-  
-  std::unique_ptr<SwitchBuilder>
-  SwitchBuilder::create(IRGenFunction &IGF, llvm::Value *Subject,
-                        SwitchDefaultDest Default, unsigned NumCases) {
-    // Pick a builder based on how many total reachable destinations we intend
-    // to have.
-    switch (NumCases + (Default.getInt() == IsNotUnreachable)) {
-    case 0:
-      // No reachable destinations. We can emit an unreachable and go about
-      // our business.
-      IGF.Builder.CreateUnreachable();
-      return nullptr;
-    case 1:
-      // One reachable destination. We can emit an unconditional branch.
-      // If that one branch is the default, we're done.
-      if (Default.getInt() == IsNotUnreachable) {
-        IGF.Builder.CreateBr(Default.getPointer());
-        return nullptr;
-      }
-      
-      // Otherwise, use a builder to emit the one case.
-      return std::unique_ptr<SwitchBuilder>(
-        new BrSwitchBuilder(IGF, Subject, NumCases));
-    case 2:
-      // Two reachable destinations. We can emit a single conditional branch.
-      return std::unique_ptr<SwitchBuilder>(
-        new CondBrSwitchBuilder(IGF, Subject, Default, NumCases));
-    default:
-      // Anything more, fall over into a switch.
-      // TODO: Since fast isel doesn't support switch insns, we may want to
-      // also support a "pre-lowered-switch" builder that builds a jump tree
-      // for unoptimized builds.
-      return std::unique_ptr<SwitchBuilder>(
-        new SwitchSwitchBuilder(IGF, Subject, Default, NumCases));
-    }
-  }
-
   /// Implementation strategy for singleton enums, with zero or one cases.
   class SingletonEnumImplStrategy final : public EnumImplStrategy {
     bool needsPayloadSizeInMetadata() const override { return false; }